End mills are commonly-used cutting tools for machining work pieces and are typically engaged to a rotary turning machine such as a milling machine. The milling machine rotatably drives the end mill to shape the work piece. End mills are typically provided as elongate, cylindrically shaped elements and may include anywhere from 2 to 20 or more teeth or flutes that are formed on an outer perimeter of the end mill. As distinguished from drill bits which are typically used for forming holes in an axial direction, end mills can be used for shaping work pieces in all directions including, without limitation, axial (i.e., vertical), lateral (i.e., sideways) and angular directions.
Each tooth of the end mill is configured to remove a small amount of material as the end mill is rotatably driven relative to the work piece. Amongst may possible shapes, end mills can have a blunt or flat end or they may be rounded with a hemispherical or semispherical end such as a ball end and may be used in CNC milling machines in order to produce a variety of different part geometries.
End mills may be engaged at one end to a chuck or collet of a spindle which may be movable in vertical, lateral and/or angular orientations depending upon the capabilities of the milling machine (i.e., whether the milling machine is 2-axis, 3-axis, 5-axis, etc.). End mills are typically fabricated of relatively hard materials such as high speed steel or tungsten carbide to provide resistance against deflection and also to maintain the integrity of the cutting tool under load. Extremely hard coatings may be formed on the cutting teeth to allow the end mill to operate under high temperature, high pressure machining conditions and to increase the life of the end mill.
In an effort to increase productivity and reduce the amount of time required to machine a work piece, the end mill may be provided with a relatively large number of cutting teeth such that the end mill may cut relative to the work piece at a high feed rate for a given chip load. Chip load is a measure of the amount of material removed by each tooth during one revolution of the end mill. In this regard, an end mill having a high tooth-count (e.g., 20 teeth) may be capable of higher feed rates for the same chip load as compared to an end mill having a relatively low tooth-count (e.g., 4 teeth).
Referring to FIG. 1, shown is a schematic illustration of a straight line tool path of a 20-tooth end mill superimposed over the tool path of a 4-tooth end mill. In the exemplary illustration, for a chip load of 0.003 inches per cutting tooth, the 4-tooth end mill has a feed distance per revolution (indicated by reference numeral 46) of 0.012 inches. In this example, the 4-tooth end mill must rotate through five revolutions in order to travel 0.060 inches. In comparison, for the same chip load (i.e., 0.003 inches), the 20-tooth end mill has a feed distance per revolution 46 of 0.060 inches and must only rotate through a single revolution to cover the same distance traveled in 5 revolutions by the 4-tooth end mill. Therefore, for the same rotational speed, the 20-tooth end mill has a higher feed rate relative to the feed rate of the 4-tooth end mill.
Unfortunately, although increased feed rates are possible, several disadvantages are associated with high tooth-count end mills. For example, radial runout is a common phenomenon associated with rotary machining operations and may be generally defined as a variation in the rotating radius of a rotating cutting tool as compared to an ideally uniform radius of rotation. Radial runout may result in the formation of surface feed marks as the end mill is driven through the work piece. Such surface feed marks may appear in machined surfaces as peaks and valleys or scallops and may require time-consuming hand-finishing operations in order to smooth out the feed marks.
Referring back to FIG. 1, shown in solid are five feed marks represented as slight arcs that are formed in the workpiece surface 48 as a result of radial runout 44 in a 4-tooth end mill as it rotates through five revolutions. In comparison, shown in dashed is a single feed mark represented as a single arc in the workpiece surface 48 by the 20-tooth end mill as a result of the same amount of radial runout 44.
Because the feed marks created by the low tooth-count (e.g., 4-tooth) end mill overlap with one another, such overlapping gives the appearance of a smoother surface profile than the surface profile created by the high tooth-count (e.g., 20-tooth) end mill when viewed without magnification. As such, the feed marks created by low tooth-count end mills are more easily disguised than feed marks created by high tooth-count end mills.
In an effort to reduce or eliminate hand finishing of parts machined by high tooth-count end mills, some manufacturers have reverted to the use of low tooth-count end mills despite the lower feed rates. Unfortunately, the use of low tooth-count end mills results in the production of machined parts at a much slower rate than would be achievable using end mills having a high tooth-count.
As can be seen, there exists a need in the art for a cutting tool or end mill which prevents the formation of feed marks in the cutting surface and thereby allows for an increase in tool feed rate in order to improve machining productivity. Furthermore, there exists a need in the art for a high tooth-count end mill which reduces or eliminates the formation of feed marks that are generated as a result of radial runout. Finally, there exists a need in the art for a cutting tool or end mill which is of low cost and which eliminates or reduces the amount of post processing or hand finishing of machined parts.